1. Field of the Invention
The present invention relates to a method for the manufacture of a poppet valve made of a .gamma.-TiAl base alloy for internal combustion engines, with valve cross sections of cylindrical parts having the thickness S, cone and shaft, in a shape which is at least close to the final dimensions of the valve shape by the primary forming of a homogeneous blank, followed by a secondary forming.
2. Description of the Related Art
Alloys made of .gamma.-TiAl base belong to the group of intermetallic materials whose properties are intermediate between those of metals and those of ceramics.
.gamma.-TiAl base alloys of industrial importance are alloys with an aluminum content of 45-48 at %, which is in practice two-phase alloys with a .gamma.-TiAl and an .alpha..sub.2 -Ti.sub.3 Al phase, or polyphase alloys, if additional alloy elements that improve the properties are added.
As additional alloy elements, it is preferred to use chromium, manganese and vanadium to increase the ductility at room temperature; niobium, tantalum, molybdenum and tungsten to increase the resistance to oxidation and to increase the rigidity; and silicon, boron and carbon to increase the thermal resistance, the creep resistance, for a targeted setting of lamellar structural types, and to improve the processability of the alloys-in-question.
These alloy elements, in total, account for approximately 1-20 at % in the .gamma.-TiAl base alloys of practical importance.
Since .gamma.-TiAl base alloys present, in addition to low density, good resistance to oxidation and good mechanical properties up to temperatures of 750.degree. C., they are increasingly used, among other applications, for blades of gas turbines and valves for internal combustion engines.
Since most intermetallic materials, including .gamma.-TiAl base alloys, present a brittle behavior at room temperature and because of the strong extensibility rate dependence of the yield stress, also at temperatures above the brittle/ductile transition temperature, the manufacture of finished products is often involved from the point of view of process technology, expensive, and strongly dependent on the forming conditions.
It is possible to manufacture the starting material by melt or powder metallurgical methods. To achieve the best possible material properties, which depend very strongly in the case of .gamma.-TiAl base alloys on particle size, phase distribution and structural homogeneity, and to achieve the desired final form, additional mechanical deformation of the starting material is often required. Superplastic forming, extrusion, drop forging or rolling, as a rule under isothermic or near isothermic conditions, have been successfully used.
DE-PS 4 318 424 describes, e.g., a method for the manufacture of formed bodies made of .gamma.-TiAl alloys, such as valves and poppet valve heads for motors. In this method, a casting was first deformed in a temperature range of 1050-1300.degree. C. under near isothermic conditions with a high degree of forming, then it was cooled, and finally it was subjected to superplastic forming until a formed part having approximately the final dimensions was obtained in a temperature range of 900-1100.degree. C., and with low extension speed of 10.sup.-4 to 10.sup.-1 /sec.
One drawback in the above-described method is that it is relatively slow, involved as far as process technology is concerned, and thus expensive.